The B[e] stars conference in Prague

During the last week I was traveling from Ondrejov forth and back to the “The B[e] Phenomenon: Forty Years of Studies”, in Prague (27 June – 1 July 2016). It was a nice conference with many interesting talks, and fruitful discussions. Most importantly, I met some old friends and made new ones! I was fortunate enough to contribute to this conference with a number of works that follow.

B[e] Supergiants are a phase in the evolution of some massive stars for which we have observational evidence but no predictions by any stellar evolution model. The mass-loss during this phase creates a complex circumstellar environment with atomic, molecular, and dust regions usually found in rings or disk-like structures. However, the structure and the formation of this circumstellar environment is not well-understood, which means that further investigation is needed. To address that, we obtained high-resolution optical and near-infrared spectra (using MPG-ESO/FEROS, GEMINI/Phoenix and VLT/CRIRES, respectively) for a number of Galactic B[e]SGs. We examined the [OI] and [CaII] emission lines and the CO bandheads to probe the structure and the kinematics of their formation regions. We find that these emission lines form either in a single or in multiple equatorial rings, a probable result of previous mass-loss events.

Despite their different evolutionary phases, B[e] supergiants and yellow hypergiants share
a number of common properties regarding their circumstellar environments. Both types of stars experience phases of strongly enhanced mass-loss, and the released material accumulates in (multiple) shells, bipolar nebulae, and/or disk-like structures, often veiling the central object. Moreover, the physical conditions in the envelopes of these stars are ideal for molecule and dust condensation. While the enhanced mass-loss and eruptions in yellow hypergiants are probably caused by an increased pulsation activity, the physical mechanism leading to the formation of the dense winds and Keplerian disks observed in B[e] supergiants is still unclear. Recently, we performed an optical spectroscopic survey of a large sample of Galactic emission-line stars in diverse evolutionary states. This survey was aimed at identifying characteristic emission features that help to study the structure and kinematics of the circumstellar environments of different types of evolved massive stars, including several yellow hypergiants and a number of B[e] stars in different evolutionary phases. Motivated by the results from previous studies, we focused on the strategic forbidden emission lines of [OI] and
[CaII], which are considered as ideal tracers for circumstellar disks. Interestingly, we identified both sets of lines in most of the yellow hypergiants in our sample, while from the B[e] star sample only the supergiants displayed these features. This indicates that the physical conditions in the environments of both types of stars (yellow hypergiants and B[e] supergiants) could be similar. In particular, the double-peaked emission lines of [CaII] observed in the yellow hypergiants of earlier spectral type suggest that these stars possibly possess a dense circumstellar ring or disk-like structure alike their hotter B[e] supergiant counterparts.

The Galactic object MWC 137 is a peculiar early-type star surrounded by the optical nebula Sh 2-266 (80″ × 60″) of unclear origin. The large-scale structure seen in Hα images suggests that Sh 2-266 is a ring nebula probably produced by the interaction of the stellar winds with the ambient medium, with a possible bipolar outflow perpendicular to the ring/disk plane. A collimated outflow with several knots was indeed recently detected in the light of the [N II] 6583 line. Moreover, near-infrared spectroscopic observations displayed intense, kinematically broadened CO band emission in both isotopes 12CO and 13CO. The observed enrichment in 13CO implies that MWC 137 is an evolved object. This result combined with the high luminosity of the star suggests that it belongs to the group of B[e] supergiants. To investigate the physical conditions and spatial distribution of the hot molecular gas we obtained K-band IFU observations with the ESO/SINFONI spectrograph in its high spatial resolution mode in two different seasons. In addition, to map the cold molecular gas regions, we collected molecular line observations in the sub-mm range with APEX. We find that the molecular gas is distributed on multiple clumpy ring structures. These rings are more or less perpendicular to the jet axis, and the material is revolving the central object on (quasi-)Keplerian orbits.

Yellow hypergiants are massive stars that have passed through the red-supergiant phase and evolve back bluewards in the Hertzsprung-Russell diagram. It has been suggested that these stars may be evolving toward the B[e] supergiant phase. Such a possible evolutionary link should be investigated.
In 2011, we started to monitor spectroscopically several yellow hypergiants using the Ondrejov 2m telescope. The aim of this campaign is to track and to study their mass ejection phases. One of the objects we monitor is ρ Cas. This star is famous for its historical and recent outbursts, during which the star develops TiO bands in a cool, optically thick wind with a very brief but high mass-loss rate (3 × 10−2 M⊙ in 200 days). Each outburst is accompanied by a drop in the light curve of more than one magnitude. At least three such outbursts were recorded for ρ Cas: 1945-1947, 1985-1986, and 2000-2001. Our spectroscopic data show that during 2013, another outburst occurred, which is obvious from the development of pronounced TiO bands. Also many atmospheric lines characteristic for a later spectral type appear. Moreover, the photometric light curve displays a drop by about 0.6 mag during the same period. While the total mass loss connected with this recent outburst was probably less violent, the decrease of the time interval between the outbursts might indicate that ρ Cas is preparing for its passage through the Yellow Void region towards the hot side of the Hertzsprung-Russell diagram.